This invention relates generally to reliability testing of electrical devices, and more particularly the invention relates to current pulse testing of electrical devices such as semiconductor components.
Semiconductor reliability tests require continuous application of electrical stimulus, usually at a controlled temperature ranging from -50.degree. C. to +350.degree. C. based on the specific test parameter (e.g., Hot Carrier, Electromigration). For electromigration test in particular, testing using DC current has always been the preferred approach due to simplicity, built-in conservatism, and relatively low cost. However, recent advances in process miniaturization have rendered DC tests insufficient, thus making similar testing under pulsed conditions a necessity.
An ideal pulsed stimulus should allow flexible control of Pulse-Repetition-Rate, Duty-Cycle, Polarity, and Intensity (Amplitude). These parameters are illustrated in FIGS. 1A and 1B where T is the period, frequency (f) is the pulse repetition rate (Hz), duty cycle is 2tp/T; positive amplitude is Ap, and negative amplitude is An (Volt, Amp). Further, while applications such as Hot Carrier (HC) and Time to Dielectric Breakdown (TDDB) require voltage stimulus, Electromigration requires a current pulse-train, which is a more difficult waveform to generate.
Conventional methods require relatively complex circuitry for each current source, as one source is needed for each DUT. While the intended use of the pulsed technique is limited to pulse-repetition-rates of several Mega-Cycle-per-Second (MHz), the required rectangular shape with a minimal overshoot and ringing, as illustrated in FIG. 2, is still difficult to meet. This requires that the pulse generating circuitry be very close to the DUT, a requirement difficult to meet especially when DUT temperatures are to be controlled up to 350.degree. C. Thus, reliability tests and electromigration test in particular require complex rectangular pulses, which are capable of driving currents below one milliampere to more than 0.1 ampere under load conditions varying from one Ohm to several kilo-Ohms, all with the above described requirements. Designing such a system based on conventionally available techniques inevitably increases circuit complexity, cost, physical size (footprint) and a compromise on pulse quality to a point where such systems become impractical.
The present invention is directed to achieving a higher quality pulsed reliability test system in which a bipolar pulse waveform is generated close to the DUT by using a dual configuration of high quality DC sources.